Anode for secondary battery, secondary battery, and method of manufacturing anode for secondary battery

ABSTRACT

An anode for a secondary battery capable of improving cycle characteristics, a secondary battery using the anode, and a method of manufacturing an anode for secondary battery. An anode active material layer is formed by vapor phase deposition method, and contains Si as an element. In the anode active material layer, there are a plurality of primary particles grown in the thickness direction. The primary particles aggregate and form a plurality of secondary particles. At least some of the primary particles have shape curved in the identical direction to an anode current collector on the cross section in the thickness direction. Thereby, stress due to expansion and shrinkage due to charge and discharge can be relaxed.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/420,276 filed May 25, 2006, the entirety of which is incorporatedherein by reference to the extent permitted by law. The applicationclaims priority to and contains subject matter related to JapanesePatent Application JP 2005-161258 filed in the Japanese Patent Office onJun. 1, 2005, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an anode containing silicon (Si) as anelement, a battery using it, and a method of manufacturing an anode.

2. Description of the Related Art

In recent years, as mobile devices have been sophisticated andmulti-functionalized, a higher capacity of secondary batteries as apower source for these mobile devices has been demanded. As a secondarybattery to meet such a demand, there is a lithium ion secondary battery.However, since the lithium ion secondary battery currently in practicaluse utilizes graphite for the anode, the battery capacity is in asaturated state and attaining a significantly high capacity thereof isdifficult. Therefore, it has been considered to use silicon or the likefor the anode. In recent years, forming the anode active material layeron the anode current collector by vapor-phase deposition method or thelike has been reported (for example, refer to Japanese Unexamined PatentApplication Publication No. H08-50922, Japanese Patent Publication No.2948205, and Japanese Unexamined Patent Application Publication No.H11-135115). Since silicon or the like is largely expanded and shrunkdue to charge and discharge, lowering of cycle characteristics due topulverization has been a problem. However, when the vapor-phasedeposition method is used, miniaturization can be inhibited, and theanode current collector and the anode active material layer can beintegrated. Therefore, electron conductivity in the anode becomessignificantly favorable, and realizing high performance in view of thecapacity and the cycle life has been improved.

SUMMARY OF THE INVENTION

However, there is a disadvantage that even in the anode in which theanode current collector and the anode active material layer areintegrated, when charge and discharge are repeated, stress is generatedbetween the anode current collector and the anode active material layerdue to intense expansion and shrinkage of the anode active materiallayer, leading to drop of the anode active material layer and loweringof cycle characteristics.

In view of the foregoing, in the present invention, it is desirable toprovide an anode for secondary battery capable of inhibiting shape decayof the anode active material layer and improving battery characteristicssuch as cycle characteristics, a secondary battery using the anode, anda method of manufacturing an anode for secondary battery.

According to an embodiment of the present invention, there is providedan anode for secondary battery in which an anode active material layercontaining silicon as an element is provided on an anode currentcollector, in which the anode active material layer has a plurality ofprimary particles grown to the anode current collector, and at leastsome of the primary particles have shape curved to the anode currentcollector.

According to an embodiment of the present invention, there is provided asecondary battery including a cathode, an anode, and an electrolyte, inwhich in the anode, an anode active material layer containing silicon asan element is provided on an anode current collector, the anode activematerial layer has a plurality of primary particles grown to the anodecurrent collector, and at least some of the primary particles have shapecurved to the anode current collector.

According to an embodiment of the present invention, there is provided amethod of manufacturing an anode for secondary battery for forming ananode active material layer containing silicon as an element on an anodecurrent collector, in which the anode active material layer is grown byvapor-phase deposition method while the incident angle of the rawmaterial of the anode active material layer is changed to the anodecurrent collector.

According to the anode for secondary battery of the embodiment of thepresent invention, the curve-shaped primary particles are included.Therefore, stress due to expansion and shrinkage due to charge anddischarge can be relaxed, shape destruction of the anode active materiallayer and peeling of the anode active material layer from the anodecurrent collector can be inhibited. Therefore, according to thesecondary battery of the embodiment of the present invention using thisanode, battery characteristics such as cycle characteristics can beimproved.

Further, according to the method of manufacturing an anode for secondarybattery of the embodiment of the present invention, the anode activematerial layer is grown by vapor-phase deposition method while theincident angle of the raw material of the anode active material layer ischanged to the anode current collector. Therefore, the anode forsecondary battery of the embodiment of the present invention can beeasily manufactured.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing a structure of a secondary batteryaccording to a first embodiment of the present invention;

FIG. 2 is an SEM photograph showing a particle structure of an anodeactive material layer according to the secondary battery shown in FIG.1;

FIG. 3 is a model view showing a particle structure of the anode activematerial layer according to the secondary battery shown in FIG. 1;

FIG. 4 shows an example of a configuration of manufacturing equipmentused in forming the anode active material layer according to thesecondary battery shown in FIG. 1;

FIG. 5 shows another example of a configuration of manufacturingequipment used in forming the anode active material layer according tothe secondary battery shown in FIG. 1;

FIG. 6 shows still another example of a configuration of manufacturingequipment used in forming the anode active material layer according tothe secondary battery shown in FIG. 1;

FIG. 7 is an exploded perspective view showing a structure of asecondary battery according to a second embodiment of the presentinvention;

FIG. 8 is a cross section showing a structure taken along line I-I ofthe secondary battery shown in FIG. 7;

FIG. 9 is an SEM photograph showing a particle structure of an anodeactive material layer according to Example 3 of the present invention;

FIG. 10 is an SEM photograph showing a particle structure of an anodeactive material layer according to Comparative example 1 relative to thepresent invention; and

FIG. 11 is an SEM photograph showing a particle structure of an anodeactive material layer according to Comparative example 2 relative to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereinafter described indetail with reference to the drawings.

First Embodiment

FIG. 1 shows a structure of a secondary battery according to a firstembodiment of the present invention. The secondary battery is aso-called coin type secondary battery in which an anode 12 contained ina package cup 11 and a cathode 14 contained in a package can 13 arelayered with a separator 15 in between. Peripheral edges of the packagecup 11 and the package can 13 are hermetically sealed by being caulkedwith an insulating gasket 16. The package cup 11 and the package can 13are respectively made of a metal such as stainless and aluminum (Al).

The anode 12 has, for example, an anode current collector 12A and ananode active material layer 12B provided on the anode current collector12A.

The anode current collector 12A is preferably made of a metal materialcontaining at least one metal element not forming an intermetalliccompound with lithium (Li). When the intermetallic compound is formedwith lithium, the anode is expanded and shrunk due to charge anddischarge, structural destruction occurs, and current collectivitycharacteristics are lowered. In addition, ability to support the anodeactive material layer 12B is lowered. In this specification, the metalmaterials include an alloy including two or more metal elements or analloy including one or more metal elements and one or more metalloidelements, in addition to simple substances of metal elements. As a metalelement not forming an intermetallic compound with lithium, for example,copper (Cu), nickel (Ni), titanium (Ti), iron (Fe), and chromium (Cr)can be cited.

The anode current collector 12A preferably contains a metal elementbeing alloyed with the anode active material layer 12B. Thereby, contactcharacteristics between the anode active material layer 12B and theanode current collector 12A can be improved. As a metal element notforming an intermetallic compound with lithium and being alloyed withthe anode active material layer 12B, in the case that the anode activematerial layer 12B contains silicon as an element as described later,for example, copper, nickel, or iron can be cited. These elements arepreferable in view of strength and electrical conductivity as well.

The anode current collector 12A may be a single layer or a plurality oflayers. In the latter case, the layer contacting with the anode activematerial layer 12B may be made of a metal material being alloyed withsilicon, and other layers may be made of other metal material. Further,the anode current collector 12A is preferably made of a metal materialmade of at least one of metal elements not forming an intermetalliccompound with lithium except for the interface with the anode activematerial layer 12B.

The face of the anode current collector 12A on which the anode activematerial layer 12B is provided is preferably roughened. Thereby, contactcharacteristics with the anode active material layer 12B can beimproved.

The anode active material layer 12B contains silicon as an element.Silicon has a high ability to insert and extract lithium, and provides ahigh energy density. Silicon may be contained in the form of the simplesubstance, an alloy, a compound, or a mixture of two or more thereof.

The anode active material layer 12B is formed, for example, byvapor-phase deposition method. The anode active material layer 12B has aplurality of primary particles formed by growing in the thicknessdirection. The plurality of primary particles aggregate and form aplurality of secondary particles.

FIG. 2 is a Scanning Electron Microscope (SEM) photograph showing aparticle structure on the cross section in the thickness direction ofthe anode active material layer 12B. FIG. 3 shows the particle structureas a model. As shown in FIGS. 2 and 3, each secondary particle 121 isseparated by a groove 122. The groove 122 is formed, for example, bycharge and discharge, and approximately reaches the anode currentcollector 12A. Each primary particle 123 is not simply adjacent to eachother, but at least some of each primary particle 123 is jointed to eachother to form the secondary particle 121.

Further, at least some of the primary particles 123 have a curved shapewith respect to the anode current collector 12A, and the primaryparticles 123 are curved in the identical direction on the cross sectionin the thickness direction, for example. Thereby, in the secondarybattery, stress due to expansion and shrinkage due to charge anddischarge can be relaxed, and expansion in the thickness direction canbe reduced. All the primary particles 123 may be curved. However, asshown in FIGS. 2 and 3, on the interface of each secondary particle 121separated by the groove 122, such a structure may be destructed in somecases. Therefore, it is not always necessary that all the primaryparticles 123 are curved in similar fashion.

Such a particle structure may be observed by SEM as shown in FIG. 2, ormay be observed by Scanning Ion Microscope (SIM). Further, the crosssection is preferably cut by FIB (Focused Ion Beam), a microtome or thelike.

The anode active material layer 12B is preferably alloyed with the anodecurrent collector 12A at least at part of the interface with the anodecurrent collector 12A. Specifically, on the interface, the element ofthe anode current collector 12A is preferably diffused in the anodeactive material layer 12B, or the element of the anode active materiallayer 12B is preferably diffused in the anode current collector 12A, orboth elements thereof are preferably diffused in each other. Thereby,even when the anode active material layer 12B is expanded and shrunk dueto charge and discharge, the anode active material layer 12B isprevented from being dropped from the anode current collector 12A.

The cathode 14 has, for example, a cathode current collector 14A and acathode active material layer 14B provided on the cathode currentcollector 14A. Arrangement is made so that the cathode active materiallayer 14B side is opposed to the anode active material layer 12B. Thecathode current collector 14A is made of, for example, aluminum, nickel,stainless or the like.

The cathode active material layer 14B contains, as a cathode activematerial, for example, one or more cathode materials capable ofinserting and extracting lithium. The cathode active material layer 14Bmay contain an electrical conductor such as a carbon material and abinder such as polyvinylidene fluoride according to needs. As a cathodematerial capable of inserting and extracting lithium, for example, thelithium-containing metal complex oxide expressed by a general formula,Li_(x)MIO₂ is preferable, since the lithium-containing metal complexoxide can generate a high voltage and has a high density, leading to ahigher capacity of the secondary battery. MI represents one or moretransition metal elements, and for example, preferably contains at leastone of cobalt and nickel. x varies according to charge and dischargestates of the battery, and is generally in the range of 0.05≦x≦1.10. Asa specific example of such a lithium-containing metal complex oxide,LiCoO₂, LiNiO₂ or the like can be cited.

The separator 15 separates the anode 12 from the cathode 14, preventscurrent short circuit due to contact of the both electrodes, and letsthrough lithium ions. The separator 15 is made of, for example,polyethylene or polypropylene.

An electrolytic solution, which is a liquid electrolyte, is impregnatedin the separator 15. The electrolytic solution contains, for example, asolvent and an electrolyte salt dissolved in the solvent. Theelectrolytic solution may contain an additive according to needs. As asolvent, for example, a nonaqueous solvent such as ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate can be cited. One of the solvents may be used singly,or two or more thereof may be used by mixing.

As an electrolyte salt, for example, a lithium salt such as LiPF₆,LiCF₃SO₃, and LiClO₄ can be cited. One of the electrolyte salts may beused singly, or two or more thereof may be used by mixing.

The secondary battery can be fabricated as follows, for example. First,the anode active material layer 12B containing silicon as an element isformed on the anode current collector 12A by, for example, vapor phasedeposition method. As vapor phase deposition method, for example,physical deposition method or chemical deposition method can be cited.Specifically, any of vacuum vapor deposition method, sputtering method,ion plating method, laser ablation method, CVD (Chemical VaporDeposition) method, and thermal spraying method may be used. Then, forexample, the incident angle of the raw material is continuously changedto the anode current collector 12A.

FIGS. 4 to 6 show an example of a structure of manufacturing equipmentof forming the anode active material layer 12B by vapor depositionmethod. For example, as shown in FIG. 4, it is possible that the anodecurrent collector 12A is attached to a fixed base 21, and the fixed base21 is moved relatively to a raw material 22. Otherwise, as shown inFIGS. 5 and 6, it is possible that while the strip-shaped anode currentcollector 12A is moved from a supply roll 23 to a winding roll 25through a support roller 24, the anode current collector 12A is movedrelatively to the raw material 22. Further, in the manufacturingequipment shown in FIGS. 5 and 6, the anode current collector 12A may bedirectly moved from the supply roll 23 to the winding roll 25 withoutthrough the support roller 24.

In particular, as shown in FIGS. 5 and 6, it is preferable that whilethe anode current collector 12A is moved through a rotational supportsuch as the supply roll 23, the support roller 24, and the winging roll25, the incident angle of the raw material is changed. Thereby, theanode active material layer 12B can be continuously formed. Further, asshown in FIG. 6, it is preferable that the anode active material layer12B is formed while the cylindrical support roller 24 is arranged in theposition opposed to the raw material 22 and the anode current collector12A is moved curvedly. Thereby, the primary particle 123 is more curvedand stress can be dispersed, the distance at which the anode activematerial layer 12B can be formed can be lengthened, and productivity canbe more improved. In FIGS. 4 to 6, the vapor deposition method has beenspecifically described. However, in other methods, the incident angle ofthe raw material 22 can be similarly changed.

Thereby, the primary particle of the anode active material layer 12B isgrown curved to the anode current collector 12A. Next, heat treatment isprovided under the vacuum atmosphere or the non-oxidizing atmosphereaccording to needs.

Subsequently, the cathode active material layer 14B is formed on thecathode current collector 14A. For example, a cathode active material,and if necessary an electrical conductor and a binder are mixed, thecathode current collector 14A is coated with the mixture, the resultantis compression-molded. After that, the anode 12, the separator 15, andthe cathode 14 are layered, the lamination is contained between thepackage cup 11 and the package can 13, an electrolytic solution isinjected, and the resultant is caulked, and thereby a battery isassembled. After the battery is assembled, for example, by performingcharge and discharge, the groove 122 is formed in the anode activematerial layer 12B, and the anode active material layer 12B is dividedinto the secondary particle 121.

In the secondary battery, when charged, for example, lithium ions areextracted from the cathode 14 and inserted in the anode 12 through theelectrolytic solution. When discharged, for example, lithium ions areextracted from the anode 12 and inserted in the cathode 14 through theelectrolytic solution. As charge and discharge are performed, the anodeactive material layer 12B is largely expanded and shrunk. However, sincethe primary particle 123 has a shape curved to the anode currentcollector 12A, stress is relaxed, and shape destruction and peeling ofthe anode active material layer 12B from the anode current collector 12Aare inhibited.

As above, according to this embodiment, at least some of the primaryparticles 123 of the anode active material layer 12B have shape curvedto the anode current collector 12A. Therefore, stress due to expansionand shrinkage due to charge and discharge can be relaxed, and shapedestruction of the anode active material layer 12B and peeling of theanode active material layer 12B from the anode current collector 12A canbe inhibited. Therefore, battery characteristics such as cyclecharacteristics can be improved.

Further, when the incident angle of the raw material to the anodecurrent collector 12A is changed, the anode 12 of this embodiment can beeasily obtained. In particular, when the anode current collector 12A ismoved through the rotational support, the anode active material layer12B can be continuously formed, and productivity can be improved.Further, when the incident angle of the raw material is changed bymoving the anode current collector 12A curvedly, the primary particle123 can be more curved to disperse stress, the distance at which theanode active material layer 12B can be formed can be lengthened, andproductivity can be more improved.

Second Embodiment

FIG. 7 shows a structure of a secondary battery according to a secondembodiment of the present invention. In the secondary battery, aspirally wound electrode body 30 on which leads 31 and 32 are attachedis contained inside a film package member 40. Thereby, a small, light,and thin secondary battery can be obtained.

The leads 31 and 32 are respectively directed from inside to outside ofthe package member 40 in the same direction, for example. The leads 31and 32 are respectively made of, for example, a metal material such asaluminum, copper, nickel, and stainless, and are in a state of thinplate or mesh, respectively.

The package member 40 is made of a rectangular aluminum laminated filmin which, for example, a nylon film, an aluminum foil, and apolyethylene film are bonded together in this order. The package member40 is, for example, arranged so that the polyethylene film side and thespirally wound electrode body 30 are opposed to each other, and therespective outer edges are contacted to each other by fusion bonding oran adhesive. Adhesive films 41 to protect from outside air intrusion areinserted between the package member 40 and the leads 31 and 32. Theadhesive film 41 is made of a material having contact characteristics tothe leads 31 and 32, for example, is made of a polyolefin resin such aspolyethylene, polypropylene, modified polyethylene, and modifiedpolypropylene.

The package member 40 may be made of a laminated film having otherstructure, a high molecular weight film such as polypropylene, or ametal film, instead of the foregoing aluminum laminated film.

FIG. 8 shows a cross sectional structure taken along line I-I of thespirally wound electrode body 30 shown in FIG. 7. In the spirally woundelectrode body 30, an anode 33 and a cathode 34 are layered with aseparator 35 and an electrolyte layer 36 in between and wound. Theoutermost periphery thereof is protected by a protective tape 37.

The anode 33 has a structure in which an anode active material layer 33Bis provided on the both faces of an anode current collector 33A. Thecathode 34 also has a structure in which a cathode active material layer34B is provided on the both faces of a cathode current collector 34A.Arrangement is made so that the cathode active material layer 34B isopposed to the anode active material layer 33B. The structures of theanode current collector 33A, the anode active material layer 33B, thecathode current collector 34A, the cathode active material layer 34B,and the separator 35 are similar to that of the anode current collector12A, the anode active material layer 12B, the cathode current collector14A, the cathode active material layer 14B, and the separator 15respectively described above. The particle structure of the anode activematerial layer 33B is determined at the central portion of the portionof the spirally wound electrode body 30 where the curvature is notlarge.

The electrolyte layer 36 is made of a so-called gelatinous electrolytein which an electrolytic solution is held in a holding body made of ahigh molecular weight compound. The gelatinous electrolyte ispreferable, since a high ion conductivity can be thereby obtained, andliquid leakage of the battery can be thereby prevented. The structure ofthe electrolytic solution is similar to of the first embodiment. As ahigh molecular weight material, for example, polyvinylidene fluoride canbe cited.

The secondary battery can be manufactured, for example, as follows.First, after the anode 33 and the cathode 34 are formed in the samemanner as in the first embodiment, the electrolyte layer 36 in which theelectrolytic solution is held in the holding body is formed on the anode33 and the cathode 34. Next, the leads 31 and 32 are attached to theanode current collector 33A and the cathode current collector 34A.Subsequently, the anode 33 and the cathode 34 formed with theelectrolyte layer 36 are layered with the separator 35 in between and iswound. The protective tape 37 is adhered to the outermost periphery ofthe lamination to form the spirally wound electrode body 30. After that,for example, the spirally wound electrode body 30 is sandwiched betweenthe package members 40, and outer edges of the package members 40 arecontacted by thermal fusion bonding or the like to enclose the spirallywound electrode body 30. Then, the adhesive films 41 are insertedbetween the leads 31, 32 and the package member 40.

Further, the secondary battery may be fabricated as follows. First, inthe same manner as in the first embodiment, the anode 33 and the cathode34 are formed. After that, the leads 31 and 32 are attached thereto.Next, the anode 33 and the cathode 34 are layered with the separator 35in between and wound. The protective tape 37 is adhered to the outermostperiphery thereof, and a spirally wound body as a precursor of thespirally wound electrode body 30 is formed. Subsequently, the spirallywound body is sandwiched between the package members 40, and theoutermost peripheries except for one side are thermally fusion-bonded toobtain a pouched state. After that, an electrolytic compositioncontaining an electrolytic solution, a monomer as a raw material for thehigh molecular weight compound, a polymerization initiator, and ifnecessary other material such as a polymerization inhibitor is injectedinside the package member 40. After that, the opening of the packagemember 40 is thermally fusion-bonded and hermetically sealed in thevacuum atmosphere. Next, the resultant is heated to polymerize themonomer to obtain a high molecular weight compound. Thereby, thegelatinous electrolyte layer 36 is formed.

After the battery is assembled as above, the groove 122 and thesecondary particle 121 are formed in the anode active material layer 33Bby, for example, performing charge and discharge in the same manner asin the first embodiment.

The secondary battery works similarly to in the first embodiment, andhas effects similar to of the first embodiment. In particular, accordingto this embodiment, expansion in the thickness direction of the anodeactive material layer 33B can be inhibited. Therefore, even when chargeand discharge are repeated, expansion of the battery can be inhibited.

EXAMPLES

Further, specific examples of the present invention will be hereinafterdescribed in detail with reference to the drawings.

Examples 1 to 3

Secondary batteries having a structure shown in FIGS. 7 and 8 werefabricated. First, the anode active material layer 33B being about 9 μmthick made of silicon was formed on the anode current collector 33A madeof a copper foil with roughened surface being 22 μm thick by electronbeam vacuum vapor deposition method. Then, in Example 1, as shown inFIG. 4, the anode current collector 33A was attached to the fixed base21, and the anode active material layer 33B was formed while the fixedbase 21 was moved to the raw material. In Example 2, as shown in FIG. 5,the anode active material layer 33B was formed while the anode currentcollector 33A was moved from the supply roll 23 to the winding roll 25through the support roller 24. In Example 3, as shown in FIG. 6, theanode active material layer 33B was formed while the anode currentcollector 33A was moved through the support roller 24 curvedly. Next,heat treatment was performed in the reduced pressure atmosphere.

Further, 92 parts by weight of lithium cobaltate (LiCoO₂) powders being5 μm in an average particle diameter as a cathode active material, 3parts by weight of carbon black as an electrical conductor, 5 parts byweight of polyvinylidene fluoride as a binder were mixed. The mixturewas put in N-methyl-2-pyrrolidone as a disperse medium to obtain slurry.Next, the cathode current collector 34A made of an aluminum foil being15 μm thick was coated with the slurry, which was dried and pressed toform the cathode active material layer 34B.

Subsequently, 37.5 wt % of ethylene carbonate, 37.5 wt % of propylenecarbonate, 10 wt % of vinylene carbonate, and 15 wt % of LiPF₆ weremixed to prepare an electrolytic solution. The both faces of the anode33 and the cathode 34 were respectively coated with a precursor solutionobtained by mixing 30 wt % of the electrolytic solution, 10 wt % ofpolyvinylidene fluoride as a block copolymer with weight averagemolecular weight of 0.6 million, and 60 wt % of dimethyl carbonate, anddimethyl carbonate was volatilized to form the electrolyte layer 36.

After that, the leads 31 and 32 were attached, the anode 33 and thecathode 34 were layered with the separator 35 in between and wound, andthe resultant was enclosed in the package member 40 made of an aluminumlaminated film. Thereby, the secondary battery was assembled.

As Comparative examples 1 and 2 relative to Examples 1 to 3, secondarybatteries were fabricated in the same manner as in Examples 1 to 3,except that in the manufacturing equipment shown in FIG. 4, the anodeactive material layer was formed while the fixed base 21 was not movedbut the position thereof was fixed to the raw material. Then, inComparative example 1, the portion located in approximately central partof the fixed base 21 was cut and used for the anode. In Comparativeexample 2, the portion located on the end of the fixed base 21 was cutand used for the anode.

For the fabricated secondary batteries of Examples 1 to 3 andComparative examples 1 and 2, charge and discharge test was conducted at25 deg C., and the capacity retention ratio at the 31st cycle to thesecond cycle was obtained. Then, charge was performed until the batteryvoltage reached 4.2 V at a constant current density of 1 mA/cm², andthen performed until the current density reached 0.05 mA/cm² at aconstant voltage of 4.2 V. Discharge was performed until the batteryvoltage reached 2.5 V at a constant current density of 1 mA/cm². Chargewas performed so that the utilization ratio of the capacity of the anode33 was 90%, and metal lithium was not precipitated on the anode 33. Thecapacity retention ratio was calculated as a ratio of the dischargecapacity at the 31st cycle to the discharge capacity at the secondcycle, that is, as (the discharge capacity at the 31st cycle/thedischarge capacity at the second cycle)×100.

Further, for the secondary batteries of Examples 1 to 3 and Comparativeexamples 1 and 2, the thickness before charge and discharge (at the zerocycle) and the thickness after repeating 31 cycles of charge anddischarge were measured and the expansion width was obtained. Then, theexpansion width ratio to Comparative example 1 was calculated accordingto Mathematical formula 1. The results are shown in Table 1.

Expansion width ratio=[(A−B)/(a−b)]×100   (Mathematical formula 1)

In Mathematical formula 1, A represents the thickness of each Examplebefore charge and discharge, B represents the thickness of each Exampleafter 31 cycles of charge and discharge, a represents the thickness ofComparative example 1 before charge and discharge, and b represents thethickness of Comparative example 1 after 31 cycles of charge anddischarge.

Further, for the secondary batteries of Examples 1 to 3 and Comparativeexamples 1 and 2, after the 31st cycle, the battery was disassembled andthe anode 33 in the discharged state was taken out, washed with dimethylcarbonate, and the cross section in the thickness direction at thecentral portion of the anode 33 was observed by SEM. The cross sectionwas cut by a microtome. FIG. 2 shown before is an SEM photograph of theanode active material layer 33 of Example 1. The SEM photograph ofExample 2 was almost the same as of Example 1. The SEM photograph ofExample 3 is shown in FIG. 9, the SEM photograph of Comparative example1 is shown in FIG. 10, and the SEM photograph of Comparative example 2is shown in FIG. 11, respectively.

As shown in FIGS. 2 and 9 to 11, in Examples 1 to 3 and Comparativeexamples 1 and 2, a state that the plurality of primary particles 123aggregate and formed the secondary particles was confirmed. Further, inExamples 1 to 3, at least some of the primary particles 123 were curvedto the anode current collector 33A. Meanwhile, in Comparative example 1,the primary particles were grown linearly and almost perpendicularly,and in Comparative example 2, the primary particles were grown linearlyat a slant.

TABLE 1 Anode active material layer Expansion Capacity manufacturingPrimary particle width retention equipment shape ratio (%) ratio (%)Example 1 FIG. 4 Curved FIG. 2 93 88.2 Example 2 FIG. 5 90 89.2 Example3 FIG. 6 FIG. 9 88 92.2 Comparative Incident angle Perpendicular 10085.2 example 1 of raw FIG. 10 Comparative material: Slant 94 86.0example 2 constant FIG. 11

Further, as shown in Table 1, according to Examples 1 to 3, theexpansion width ratio could be small and the capacity retention ratiocould be improved compared to Comparative examples 1 and 2. That is, itwas found that when at least some of the primary particle 123 had acurved shape to the anode current collector 33A, direction of expansionand shrinkage due to charge and discharge could be dispersed, batterycharacteristics such as cycle characteristics could be improved, and theexpansion ratio of the device could be small.

In particular, Example 3 could provide the most superior characteristicsamong the examples. It is thinkable that in Example 3, the primaryparticle was more curved and stress could be more dispersed compared toExamples 1 and 2. That is, it was found that the anode active materiallayer 33B was more preferably formed by changing the incident angle ofthe raw material while the anode current collector 33A was movedcurvedly.

While the present invention has been described with reference to theembodiments and the examples, the present invention is not limited tothe foregoing embodiments and the foregoing examples, and variousmodifications may be made. For example, in the foregoing embodiments andthe foregoing examples, descriptions have been given of the case usingthe electrolytic solution as a liquid electrolyte or the so-calledgelatinous electrolyte. However, other electrolyte, a solid electrolytehaving ion conductivity, a mixture of a solid electrolyte and anelectrolytic solution, or a mixture of a solid electrolyte and agelatinous electrolyte, may be used.

For the solid electrolyte, for example, a high molecular weight solidelectrolyte in which an electrolyte salt is dispersed in a highmolecular weight compound having ion conductivity, or an inorganic solidelectrolyte composed of ion conductive glass, ionic crystal or the likecan be used. As a high molecular weight compound of the high molecularweight solid electrolyte, for example, an ether high molecular weightcompound such as polyethylene oxide and a cross-linked body containingpolyethylene oxide, an ester high molecular weight compound such as polymethacrylate, or an acrylate high molecular weight compound can be usedsingly, by mixing, or by copolymerization. As an inorganic solidelectrolyte, a substance containing lithium nitride, lithium phosphateor the like can be used.

Further, in the foregoing embodiments and the foregoing examples,descriptions have been given of the coin-type secondary battery and thespirally wound laminated type secondary battery. However, the presentinvention can be similarly applied to a secondary battery having othershape such as a cylinder type secondary battery, a square type secondarybattery, a button type secondary battery, a thin secondary battery, alarge secondary battery, and a laminated type secondary battery.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alternations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A secondary battery comprising: a cathode; ananode; and an electrolyte, wherein, the anode comprises an anode currentcollector and an anode active material layer containing silicon (Si) asan element on the anode current collector, the anode active materiallayer has a plurality of primary particles grown in one layer on theanode current collector and in contact with the anode current collector,at least some of the plurality of primary particles are adjacent andjoined to each other along a direction parallel to a surface of theanode current collector and aggregate to form a secondary particle, saidanode active material layer has a plurality of distinct secondaryparticles, respective grooves are formed between adjacent secondaryparticles separating the adjacent secondary particles from each other,each groove being wider at an opening of the groove than at a bottom ofthe groove, the bottom of the groove comprising a surface of the anodeactive material layer, each secondary particle comprises more than twoprimary particles that are joined and curved in an identical directionwith respect to each other when viewed in cross section of the secondarybattery along a thickness direction of the anode current collector, andthe anode has a region including at least one groove and at least oneset of adjacent secondary particles.
 2. The secondary battery accordingto claim 1, wherein the primary particles having the curved shape arestructured such that the primary particles are effective to inhibit ashape destruction of the anode active material layer and a peeling ofthe anode active layer from the anode current collector.
 3. Thesecondary battery according to claim 1, wherein the anode currentcollector comprises a metal material alloyed with silicon.
 4. Thesecondary battery according to claim 1, wherein the anode currentcollector comprises a plurality of layers, the plurality of layersincluding a first layer that contacts the anode active material layer,the first layer comprising a metal material alloyed with silicon.
 5. Thesecondary battery according to claim 1, wherein the bottom of the grooveis closer to a side of the anode active material layer that is adjacentto the anode current collector than to a side of the anode activematerial that is opposite to the anode current collector.